Re: requestAnimationFrame

On Tue, Nov 16, 2010 at 12:03 PM, Gregg Tavares (wrk) <gman@google.com>wrote:

> One is, how should this api be used if I want an app to update at 10hz.  It
> seems to be designed to assume I want the maximum frame rate. If I want to
> run slower would I just use
>
> setInterval(function() {
>    window.requestAnimationFrame();
>   }, 100); // request frames at 10hz?
>
> That's fine if that's the answer
>

If you really want to animate in 10Hz steps, then I suggest you do something
like
var start = window.animationTime;
var rate = 10; // Hz
var duration = 10; // s
var lastFrameNumber;
function animate() {
  var elapsed = window.animationTime - start;
  if (elapsed < duration) {
    window.requestAnimationFrame(animate);
  }
  var frameNumber = Math.round(elapsed/(1000/rate));
  if (frameNumber == lastFrameNumber)
    return;
  lastFrameNumber = frameNumber;
  // ... update the display based on frameNumber ...
}
window.requestAnimationFrame(animate);

(Out of curiosity, what are the use-cases for this?)

That seems to have 2 issues.
>
> 1) All of them will get a beforePaint event even if most or all of them are
> scrolled off the visible area since there is only 1 beforePaint event
> attached to the window.
>

Something could be done here, but it seems rather complex to specify things
in a way that avoids firing callbacks when relevant content is "out of
view".

2) All of them will get beforePaint events at the speed of the fastest one.
> If one ad only needs to update at 5hz and other updates at 60hz both will
> update at 60hz.
>

The above approach solves this I think.

Rob
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